The present invention relates to a system for controlling the rotational speed of a hydraulically driven cooling fan for an internal combustion engine, and more particularly relates to such a system for controlling the rotational speed of a hydraulically driven cooling fan for an internal combustion engine according to the temperature of the propelling hydraulic fluid that is being used for hydraulically driving said cooling fan as well as according to the temperature of the coolant of said internal combustion engine.
It is known from, for example, Japanese Utility Model Publication No. 49-40183 (1974) and Japanese Patent Laying Open Publication No. 58-13119 (1983) to propel a cooling fan for an internal combustion engine of an automotive vehicle by hydraulic fluid under control of the rotational speed thereof by varying the rate of flow of such propellant hydraulic fluid to a hydraulic motor which drives said cooling fan, in such a way as to increase said rotational speed along with increase of the temperature of the coolant fluid for the internal combustion engine.
However, when the temperature of the propellant hydraulic fluid for the hydraulic motor varies as is inevitable, the efficiency of the hydraulic motor varies, and as a result the rotational speed of said cooling fan is varied. Thus, even if the flow rate of said propellant hydraulic fluid is controlled depending upon the temperature of the coolant of the internal combustion engine, when the propellant hydraulic fluid is at a relatively high operating temperature the rotational speed of the cooling fan may fall short of its target rotational speed, thereby engendering a risk of poor cooling of the internal combustion engine. In the contrary case, when the propellant hydraulic fluid is at a relatively low operating temperature, the rotational speed of the cooling fan may exceed its target rotational speed, thereby engendering a risk of over cooling of the internal combustion engine and causing an annoying fan noise, as well as reducing overall engine efficiency.